Work machines provided with an engine-driven implement include, for example, a work machine in which the load applied to the implement increases according to work conditions, such as in a walk-behind lawnmower. A walk-behind lawnmower is a work machine that cuts grass using a cutter while under self-propulsion, and that is controlled by a walking operator. This type of walk-behind lawnmower is described in Japanese Patent Laid-Open Publication No. 9-301015 (JP 09-301015 A).
The walk-behind lawnmower disclosed in JP 09-301015 A cuts grass while under self-propulsion by using the output of an engine to rotate a drive wheel and a cutter used for cutting grass. An operator operates a running clutch lever and switches the running clutch lever between a disengaged state and an engaged state, whereby the drive wheel switches between a stopped state and a running state. At this time, the angle of a throttle valve in the engine is temporarily reduced in conjunction with the switching action of the running clutch lever. As a result, since the engine speed is temporarily reduced, travel by the walk-behind lawnmower is smoothly started and stopped in a low-speed state.
Examples of lawnmowers, mowing machines, and various other types of engine-driven work machines in which a throttle valve is automatically controlled by an electronic governor provided to the engine have recently been disclosed in Japanese Patent Laid-Open Publication Nos. 4-350333 (JP 04-350333 A) and 2005-98223 (JP 2005-098223 A).
In the lawnmower, mowing machine, or other engine-driven work machine disclosed in JP 04-350333 A, an electronic governor automatically controls the angle of a throttle valve according to the size of a load when the operator moves a throttle lever to perform work. The engine-driven work machine is also configured so that the throttle valve is automatically closed completely or partway when the operator removes his hand from the throttle lever and stops the work operation. As a result, the engine speed returns to an idling speed or a certain minimum speed.
The walk-behind lawnmower disclosed in JP 2005-098223 A is provided with an electronic governor, and cuts grass while under self-propulsion by using the output of an engine to rotate a drive wheel and a cutter used for cutting grass. The electronic governor controls the engine speed as described in (1) through (3) below by electrically controlling the opening and closing of the throttle valve.
(1) When the operator is performing neither the operation for traveling nor the operation for grass cutting, the electronic governor performs control so that the actual engine speed (actual RPM) is adjusted to an idling speed.
(2) When the operator is performing only the operation for traveling, the electronic governor performs control so as to gradually increase the actual engine speed. As a result, sudden movement of the walk-behind lawnmower is prevented.
(3) When the operator is performing the operation for grass cutting, the electronic governor performs control so that the actual engine speed is kept high after being increased without interruption to a target engine speed used during grass cutting, regardless of whether the operation for traveling is being performed.
The work conditions that are in effect during operation of the engine-driven work machines disclosed in JP 9-301015 A, JP 4-350333 A and JP 2005-98223 A are not necessarily constant. According to the work conditions, the load placed on the implement can vary significantly during work. The load placed on the engine can significantly vary as a result.
Since the electronic governor electrically controls the opening and closing of the throttle valve so that the actual engine speed conforms to the target engine speed, the angle of the throttle valve is increased according to the increase in load when the load on the engine increases.
However, when the load on the engine is at maximum, the actual engine speed begins to decrease with respect to the target engine speed even though the angle of the throttle valve has been increased. Since the rotation of the grass cutter decreases when the actual engine speed decreases, the efficiency of the grass cutting work decreases.
It is also possible for the operator to move the throttle lever to appropriately adjust the engine speed according to the engine load. The engine output is adequate even when the machine is operated in a state in which the engine speed is reduced when the load is small. When the load is large, the machine may be operated in a state in which the engine speed is increased to increase the engine output.
However, it is troublesome for the operator to move the throttle lever and adjust the engine speed to the proper value each time the load on the engine changes during work. When the engine speed is reduced to an excessive degree, the output of the engine becomes too low to handle the load. A certain level of experience is thus required to appropriately perform fine adjustment of the engine speed.
The engine may be maintained in a state of high-speed rotation so that the output of the engine can be kept high in order to obviate this troublesome adjustment operation that requires experience. However, when such a configuration is adopted, the engine speed remains high even when the load is small. As a result, the noise created by the engine during high-speed rotation continues, which is disadvantageous for improving the work environment. This configuration is also disadvantageous with regard to reducing fuel consumption by the engine.
There is therefore a need for a technique whereby the ease of operation and the working efficiency of an engine-driven work machine can be increased by enabling the operator to conveniently adjust the engine speed. There is also a need for a technique whereby the noise generated by an engine-driven work machine can be further reduced, and the working environment can be further improved.